The present invention relates to a headbox for a machine for the manufacture of fibrous webs made from a stock suspension, and more particularly for the manufacture of paper webs. The headbox is of the type having a machine wide outlet duct bordered by two duct walls, respectively located above and below, and converging in the direction of flow of suspension to form an outlet slot in the downstream region. One duct wall is movably supported, and is preferably pivotally supported at its upstream end, so that the clear width of the outlet slot can be varied by means of a lifting appliance. A beam extends across the width of the machine adjacent to the adjustable duct wall. A back pressure cushion is disposed between the duct wall and said beam.
Headboxes of this type are known from German Patent Specification No. 1,461,176 and from Austrian Patent Specification No. 309,973, which corresponds to British Patent Specification No. 1,361,083.
A major problem with such headboxes is keeping the clear width, i.e. the height between the duct walls, of the outlet slot uniform with the greatest possible accuracy over the entire machine width. In practice, it has been shown that local deviations in the desired outlet slot width impairs the quality of the resulting paper web Investigations have shown that a W-shaped or M-shaped profile of the cross section of the outlet slot is usually found, which produces a corresponding irregular "weight per unit area transverse cross section" (usually called "cross direction profile of the basis weight") of the paper web It has also been shown that a certain error in the slot width of the outlet slot can cause a ten fold error in the basis weight of the paper web.
A further difficulty is that certain changes in slot width occur during the operation of the paper making machine and these do not usually disappear altogether. Such temporary, normally local, changes in slot width are caused in particular by fluctuations in the temperatures inside the paper making machine, on the one hand, and in the ambient environment, on the other hand. Fluctuations in temperature inside the paper making machine occur especially if the production cycle is interrupted, because in that case, different machine parts and the stock suspension have different temperatures. The temperature of the stock suspension is normally between 30.degree. C. and 60.degree. C., or more, depending upon the grade of paper. In operation, the interior of the adjustable duct wall adapts to this stock temperature, while the exterior of that wall and the beam are exposed to a fluctuating ambient temperature. This causes thermal stresses, from which undesirable deformations of the wall result.
With the headboxes specified in German Patent Specification No. 1,461,176 and Austrian Patent Specification No. 309,973, the adjustable duct wall, which determines the outlet slot, and to which the beam is rigidly attached, is supported at both axial ends, i.e. at the front side and at the drive side of the machine, on the lifting appliance. Fixed end moments, which also have an undesirable effect on the contours of the movable duct wall, result from this type of support. This occurs because the loading of the movable duct wall originating from the pressure of the stock suspension is substantially uniformly distributed over its length, i.e. over the machine width. However, in the opposite direction, the loading of the movable duct wall is composed of a uniformly distributed load produced by the back pressure cushion and from single loads, produced by the lifting appliance, and from the aforementioned fixed end moments. Thus, it is not possible to obtain satisfactory results with the back pressure cushion alone.
Repeated attempts have been made to improve the uniformity of the outlet slot width by additional measures. More particularly, a deformable component, e.g. a profile bar, is provided at the outlet slot. This can be adjusted by means of a plurality of spindles which are regularly spaced over the machine width. However, it is again not possible to obtain with sufficient accuracy a rectilinear contour of the edge of the movable duct wall limiting the outlet slot across the full machine width. In any case, the accuracy previously attained is frequently not sufficient to meet current requirements for paper quality.